This variable is only used if running directly off of a git checkout, either MediaWiki core or any of its extensions. Most users do not do this, and hence should not worry if git is unavailable. If the git binary is not present at the specified location, you will not be able to see git hashes on Special:Version (and you will see a PHP warning). You can either install git in that location and adjust $wgGitBin accordingly or set $wgGitBin = false; to avoid the warning.

If you however do run a git checkout (meaning you have a .git folder in your installation directory), but Git is not available, then you should set $wgGitBin = false;. Otherwise you will get errors on top of the "Special:Version" page telling you that the according binary would not have been found.